1. Field of the Invention
The present invention relates to ore processing and more particularly to methods for reducing consumption of cyanide.
2. Description of the Related Art
The mining industry has for many years utilized cyanide as an agent to extract gold from ore. Environmental concerns, however, have made the use of cyanide relatively expensive, particularly in view of the controls necessary to prevent rogue emissions of cyanide into the environment. As a consequence, extensive efforts have been carried out, in some cases to control the emissions of cyanide, and in others to find feasible alternatives to cyanide. However, cyanide remains a dominant reagent in the extraction of gold, especially in those ores in which the gold is relatively difficult to extract using other methods.
A recent development in mining extraction techniques is known by the trademark BIOX(copyright), which is carried out on refractory ores or concentrates prior to cyanidation. A refractory gold ore is one that does not respond well to the cyanidation technique. One of the reasons for refractoriness is that the gold particles are encapsulated in an inert matrix, such as a sulfide mineral, which is impervious to the cyanide leach solution. The BIOX(copyright) process makes use of bacteria which oxidize certain portions of an ore sample, such as sulfide minerals, in order to make gold, or other precious elements, more accessible to reagents such as cyanide.
In one application, the BIOX(copyright) bacteria oxidize a sulfide-bearing ore or concentrate, and as a consequence the process converts a significant proportion of the sulfide therein from the sulfide form to a stable sulfate form. Conveniently, the resulting sulfate form is unreactive to cyanide. However, during the bacterial oxidation process, a measurable percentage of elemental sulfur is usually formed as well as other intermediate sulfur bearing species, and these species are reactive to cyanide. Therefore, when the BIOX(copyright) pretreated material is passed through the cyanidation process, there tends to be a significantly higher consumption of cyanide than is necessary simply to extract the gold or other precious metals from the ore sample. Another recent bacterial leaching development in mining extraction techniques is known as the BACTECH process, which is also carried out on refractory ores or concentrates prior to cyanidation, with the same objective as the BIOX(copyright) process.
Other pretreatment processes that have been developed to oxidize refractory gold ores and make them amenable to cyanidation, and which are also known to produce elemental sulfur and other intermediate sulfur compounds that are reactive in cyanide solution, are the mild pressure leaching processes that are known by the trademarks ACTIVOX and CESL, as well as the high temperature, atmospheric pressure leaching processes, which use ferric ions or cupric ions for the oxidant.
Thus, although the BIOX(copyright) process and the alternative pretreatment processes have given significant benefits to the mining industry, it would nonetheless be desirable to reduce the consumption of cyanide in the processes following them.
It is an object of the present invention to provide a novel processing technique.
It is a further object of the present invention to provide a processing technique to reduce cyanide consumption in mineral processing operations.
It is a further object of the present invention to provide a processing technique to reduce thiocyanate formation in precious metals processing operations.
Briefly stated, the invention involves a method for reducing cyanide consumption in precious metals mining extraction processes involving a sulfur bearing ore body, comprising the steps of:
providing a precious metal bearing material having intermediate sulfur oxidation products, and
exposing the material to a solution containing sulfite ions, bisulfite ions or sulfur dioxide in a manner to cause at least a portion of the intermediate sulfur oxidation products to be converted to species that are either unreactive with cyanide or are soluble and can be washed from the precious metal bearing material.
In another aspect of the present invention, there is provided a precious metal bearing material from a precious metals mining extraction process, comprising intermediate sulfur oxidation products, most of which are unreactive with cyanide.
In another aspect of the present invention, there is provided a method for recovering precious metals from a sulfur-bearing ore body, comprising the steps of:
providing a precious metal-bearing material containing intermediate sulfur oxidation products;
exposing said material to either sulfur dioxide gas or sulfite ions or bisulfite ions, with or without added air or oxygen gas, in a manner to cause at least a portion of said intermediate sulfur oxidation products to be rendered unreactive with cyanide; and
reacting said material with cyanide to recover precious metal from said material.
In still another aspect of the present invention, there is provided a method for recovering precious metals from a sulfur-bearing ore body, comprising the steps of:
providing a precious metal-bearing material containing intermediate sulfur oxidation products;
exposing said material to either sulfur dioxide gas or sulfite ions or bisulfite ions, with or without added air or oxygen gas, in a manner to cause at least a portion of said intermediate sulfur oxidation products to be rendered unreactive with cyanide; and thereafter;
reacting said material with cyanide to recover precious metal from said material.
In still another aspect of the present invention, there is provided a method for recovering precious metals from a sulfur-bearing ore body, comprising the steps of:
providing a precious metal-bearing material containing intermediate sulfur oxidation products;
exposing said material to either sulfur dioxide gas or sulfite ions or bisulfite ions, with or without added air or oxygen gas, and
reacting said material with cyanide to recover precious metal from said material,
wherein the exposing step is conducted under conditions sufficient to reduce the content of intermediate sulfur oxidation products which are reactive to cyanide, thereby reducing the consumption of cyanide in the reacting step.